1. Field of the Invention
This invention relates to an optical scanning device, and in particular to an optical scanning device suitable for use in an apparatus such as a color laser beam printer having an electrophotographic process which is adapted to optically scan a surface to be scanned which is an image carrier by the use of a plurality of laser beams.
2. Related Background Art
Heretofore, in the optical scanning device of a color laser beam printer (a color LBP) or the like, the surface of an image carrier has been optically scanned by the use of a plurality of laser beams to thereby effect reading-in or reading-out of an image.
For example, in U.S. Pat. No. 4,253,102, as shown in FIGS. 1A and 1B of the accompanying drawings, a substrate 53 is rotated by an angle .theta. in the direction of arrow by the use of a so-called monolithic semiconductor laser having a plurality of light emitting portions 51 and 52 provided on the substrate 53, whereby the pitch in the sub scanning direction perpendicular to the main scanning direction is changed to thereby adjust the line density of writing-in. That is, in FIG. 1A, the pitch A of the light emitting portions 51 and 52 is made into a pitch B by rotating the substrate 53 by the angle .theta.. This method has suffered from the problem that it is difficult to adjust the interval between a plurality of light beams spaced apart from one another and the laser beam does not enter symmetrically with respect to a scanning system and therefore bending of the scanning line on the scanning surface and curvature of the image field occur and optical aberration becomes asymmetrical between light beams.
In Japanese Laid-Open Patent Application No. 61-15119, as shown in FIG. 2 of the accompanying drawings, laser beams from two laser oscillators 61 and 62 are collimated by collimator lenses 63 and 64, respectively, whereafter they are directed to a half prism 65 and combined together, and directed to a light deffector, not shown, to thereby accomplish optical scanning.
This method has suffered from the problem that where the pitch in the sub scanning direction is to be adjusted finely, the adjustment of the directions of the laser beams is difficult and therefore the adjusting mechanism becomes complex and the interval between the plurality of laser beams is fluctuated by the fluctuation of the environment.
Japanese Laid-Open Patent Application No. 57-22218 discloses an optical scanning device as shown in FIG. 3 of the accompanying drawings wherein use is made of a semiconductor laser array comprising a plurality of light emitting sources (10a, 10b) arranged in a direction parallel to a joined surface. the direction of arrangement of said array is disposed so as to be orthogonal to the plane of deflection of the light beams and use is made of the plurality of light beams from said semiconductor laser array which scan adjacent scanning lines 10a' and 10b' at a time using a plurality of beam spots arranged in a direction orthogonal to the scanning direction of said light beams. The light beams pass through an objective lens 12 of a rotation-symmetrical system, an anamorphic lens 11 having different refractive powers in directions orthognal to each other, a rotatable polygonal mirror 13, a rotation-symmetrical lens 14, and an anamorphic lens having different refractive powers in directions orthogonal to each other. Also, in this optical scanning device, the imaging magnification of the imaging optical system from the laser array to the surface to be scanned in the plane of deflection of the light beams is made greater than the imaging magnification in a plane orthogonal to the plane of deflection of the light beams and containing the optic axis of the imaging optical system. However, but no mention is made of the magnitude of the imaging magnification in the plane orthogonal to the plane of deflection of the light beams and containing the optic axis of the imaging optical system. Here, the plane of deflection refers to the light beam plane formed with the lapse of time by the light beams deflected by the deflecting-reflecting surface of a deflector.
Generally, when considering the stability against the fluctuation of the environment, it is better to provide the plurality of light emitting portions on a monolithic element. In this case, it is better to make the distance between the light emitting portions great to a certain extent to avoid, for example, thermal and electrical interferences therebetween in order to permit the light emitting portions to operate independently of each other. In many cases, this distance is of the order of 50 .mu.m to 100 .mu.m.
On the other hand, in many optical scanning devices, the light emitting portion of the laser oscillator and the scanning surface are maintained in substantially conjugate relationship. The lateral magnificant at this time is generally set to the order of .+-.10-40 times from the viewpoint of the utilization efficiency of the laser beam, the spot diameter and the size of the entire device.
Accordingly, when the interval between the laser beams in the sub scanning direction is 100 .mu.m, the interval on the scanning surface is on the order of 1-5 mm.
In the ordinary LBP, the adjacent line densities must be of the order of 0.2-0.04 mm and therefore, as previously described, the substrate had to be rotated by a predetermined angle to thereby narrow the pitch of the light emitting portions.
This has led to the problem that, as previously noted, bending of the scanning line and curvature of image the field occur to reduce the optical performance.